The present invention relates to expansion anchors for securing rock bolts in drill holes in mines or other rock formations, and more specifically to novel expansion anchors suitable for use in drill holes of more than one nominal diameter, i.e., over a wider range of drill hole diameters.
Expansion anchors are among the more common means of firmly securing rock bolts within drill holes in rock formations so that the bolt may be tensioned against a bearing plate engaging the rock surface surrounding the hole, thereby stabilizing the rock formation. Such anchors conventionally include an expansion shell which is forced radially outward into gripping engagement with the wall of the drill hole by advancement of a tapered nut axially into the shell. The nut is advanced by rotation of the rock bolt with which it is threadedly engaged.
In some prior expansion anchors both the external surface of the nut, or wedge, and the opposing internal surface of the shell are tapered toward the central axis of the anchor. It is also the usual practice to provide means for retaining the shell and nut in assembled relation prior to use, one of the most common of such means being a bail or strap engaging portions of the shell on each side and extending over the large end of the nut, the small end being inserted into the upper end of the shell. A standard rock bolt is threaded into a tapped hole through the central axis of the tapered nut and inserted into a drill hole which has been formed in an upper or side wall of a mine tunnel or other rock formation with the assembled expansion anchor supported on the end of the bolt which is inserted into the hole. The maximum transverse dimension of the anchor assembly must, of course, be no larger than the drill hole diameter; at the same time, however, the outer dimensions of the anchor cannot be significantly smaller than the drill hole diameter or the anchor will simply rotate with the bolt rather than being expanded into engagement with the drill hole wall, and/or will fail to attain the necessary holding force after full expansion.
In order to meet the rather stringent dimensional parameters required to insure the desired operation of the anchors, it has been necessary to form the drill holes wherein a particular anchor is to be used within 1/32" on either side of a nominal diameter. For example, conventional expansion anchors in use at the present time which are intended for use in drill holes having a nominal diameter of 11/4" will operate satisfactorily over a range of actual drill hole sizes from 1.218" to 1.281", or a total range of drill hole size of 0.063". This, of course, requires frequent replacement of drill bits since a relatively small amount of wear results in a drill hole size in which the designated expansion anchor will not operate satisfactorily. Also, it is necessary to provide a different expansion anchor for use in drill holes made with bits of nominal sizes only 1/8" apart. Thus, it has been necessry for mines to stock two different and separate models (sizes) of expansion anchors for use in nominal 11/4" holes and in 13/8" holes. The aforementioned dimensional requirements of the anchors, however, has heretofore prevented the use of a single model of expansion anchor in drill holes of more than one nominal size with a tolerance from that nominal size on the order of + or -1/32".
It is a principal object of the present invention to provide a novel and improved rock bolt expansion anchor which will operate satisfactorily in drill holes over a range of diameters approximately three times that in which prior expansion anchors would satisfactorily operate.
Another object is to provide an expansion anchor which operates in the same general manner as prior anchors, i.e., by axial advancement of a tapered nut into a hollow shell by rotation of the rock bolt, and does not add significantly, if at all, to the cost of prior anchors, yet will operate satisfactorily in drill holes having nominal diameters 1/8" apart.
A further object is to provide a rock bolt expansion anchor which reduces the number of different models or sizes of such anchors which must be stocked by an end user for operation in various size drill holes. p Still another object is to decrease the frequency of changing and sharpening drill bits in mining and similar operations where holes are drilled in rock formations for the insertion of rock bolts with expansion anchors supported thereon.
Other objects will, in part, be obvious and will, in part, appear hereinafter.